Gas gage and temp gage not working right and service engine light

assuming you didn't put thermostat in upside down, your system may need to be flushed. if radiator is blocked you can feel a cold spot at the blockage when the motor is warm

Head gasket most likely. If you drove it too long while hot, the heads may have warped. It's gonna be expensive.

It's very possible that the gauge itself is bad, it is common for the stepper motor that drives the gauge to go bad.

Don't worry about the fan. I think you have a bad temp sender unit. Replace that and see if it helps the stalling also. If not, go for an engine analysis. They will check for trouble codes and check engine sensor functions.

run the fault codes
indicates a loose ground wire , probably the battery terminal loose or dirty

My 2001 Jetta GLS Check engine light is on and I am getting 2 trouble codes P0171 and P1296. What does this mean? My temp gage is also reading nothing at all. 1 more thing I noticed when my low fuel light came on the other day my temp gage began to work but when I filled up with gas the temp gage went to zero again.

P1296 indicates that the car isn't warming up fast enough or at all. Determine if the gauge is faulty by checking the temp of the engine with an infrared thermometer. A healthy engine should operate at 190-200 deg. F fully warmed up. It may be that the gauge is accurate and your engine isn't reaching optimal temp because of a faulty thermostat. It also could be that the engine is actually warm and the gauge is wrong. Use the search feature, we've gone over this one a few times. Also note that P1296 has been mis defined a few times too. P0171 has been discussed here a million times. Check the trouble codes section and the search box.
,,,,, I pulled this off of the OBD home page, It is a close shot for your car, Hope it helps.

you might need a new thermostat

It sounds to me like you're low on coolant. When you checked the coolant level, did you actually open the radiator cap and make sure the radiator is full? The plastic tank you generally add coolant too is actually an 'overflow' or 'reserve' tank. There is a hose that runs from that tank to the top of the radiator so if the radiator gets low on coolant, more coolant -should- run in from the overflow tank. However in practice this does not always work. I have a Jeep that leaks coolant and experiences this exact same problem. Coolant will not run from the reserve tank into the radiator because the hose is plugged up, and whenever the radiator gets low on coolant the temp gauge will go real high and then drop, and the heater doesn't work well. This is because the cooling system of your car is supposed to be a closed system, full of coolant and no air. When coolant leaks out, the space it used to occupy is now occupied by air, which does not transfer heat well. When 'air' is passing through your cooling system, no heat can be transferred from your engine to the heater and radiator, resulting in a hot engine and no heat at the heater. Then when a pocket of water passes through the system, the temperature gauge quickly falls as the water absorbs the heat from the engine. The hot water that cools the engine is where the heater gets it's heat from as well, so when water passes through the heater core, the heater works, but when it's filled with air, it doesn't.

U have a problem with the air temp control blend door actuator motor, very common failure. In order to replace this part the dash must come out.

should be fine as long as the gage goes up and down and you get heat it should be fine make sure you consider the outside temp. as the engine starts it will heat up get really hot the the thermostate opens and the gage will fall and should but not always stay low it could just be the ip gage.